POWERGRID experience on insulation co-ordination of high voltage substations located at high terrain and snow bound area

To meet the power requirement of Sikkim which is located at very tough hilly terrains with heavy rains & snow bounded area, a comprehensive scheme for strengthening of transmission, sub transmission and distribution system which includes new substation as well as extension in existing substation network, was evolved by Govt. of India in consultation with POWERGRID. The inception phase of the project included assessment of-techno commercial issues however, during detailed engineering of substation works (up to 220kV), multiple-technical challenges were encountered, majorly, Insulation co-ordination of substations situated at an altitude >1000 m which further vary up to the highest altitude level of 2555 m.

This paper shares POWERGRID’s experience during detailed engineering of Sikkim project where substation-locations are at very high altitude i.e., >1000 m and explains factors considered for insulation co-ordination of-equipment i.e., circuit breaker, isolators, CT, PT, insulator strings, cable termination kits etc. at high altitude. Out of total 29 numbers of substations includedin the Sikkim project, 09 numbers of substations were situated at an altitude >1000 m. Dielectric strength of the equipment was calculated with atmospheric correction factor as specified in IEC 60071-2 [2] and dielectric test requirement as per IEC 60060-1 [3] which categories its-rated insulation levels.With varying correction factors in accordance with the altitude, all the required, statutory-air clearances i.e., Ph-Ph, Ph-E and section clearances were also corrected with altitude correction factor for-substations situated at altitude >1000 m which have impact on the overall layout finalization of the substation.

Various challenges arisen due to above insulation co-ordination like special impulse type testing requirement, constraints faced by manufacturers in non-standard dielectric testing, increased air gap requirements due to decreased insulating property of air with altitude etc. Due to multiple levels within the switchyard due to hilly-terrain with huge level difference (high slope) it was very challenging to check and achieve all altitude corrected-clearances which further varies from one substation to other located at different altitude.